The present invention relates to a base isolator capable of cutting off or reducing the shake or vibration of a substructure due to an earthquake or the like by means of mutually eccentric rotators, wherein even though the shake of the substructure is transmitted to a superstructure, the base isolator is capable of restoring the superstructure into its original position in which the superstructure has been located before the occurrence of the shake, this base isolator being appropriate for use in a location where, for example, a building, an atomic machine, a measuring instrument, a computer, a showcase or the like is placed.
FIG. 12 shows a sectional view of an essential part of a prior art base isolation mechanism utilizing eccentric rollers. It has been known that a base isolation structure utilizing rotators such as rollers or balls has an excellent base isolation effect of preventing the shake of a substructure from being transmitted to a superstructure. The base isolation structure shown in FIG. 12 has been developed by Okumura Corporation for the purpose of achieving an excellent restoration characteristic in addition to the above-mentioned base isolation function (Japanese Patent Publication No. SHO 62-32300).
As shown in FIG. 12, in this base isolator, a middle plate 5 is interposed between an upper plate 2 fixed to a lower surface of a superstructure 1 and a lower plate 4 fixed to an upper surface of a substructure 3. An upper roller bearing 6 interposed between the upper plate 2 and the middle plate 5 and a lower roller bearing 7, interposed between the lower plate 4 and the middle plate 5 diagonally to the upper roller bearing 6.
The upper roller bearing 6 and the lower roller bearing 7 have an identical structure, in which small-diameter rollers 8 and a large-diameter roller 9 which protrudes radially outwardly from the circumference of each small-diameter roller 8 are formed as a unitary member. The small-diameter rollers 8 and the large-diameter roller 9 are eccentric to each other, and the center P2 of the large-diameter roller 9 is located vertically below the center P1 of the small-diameter rollers 8 in static positions of the roller bearings 6 and 7. As a result, the quantity of protrusion of the large-diameter roller 9 in the radially outward direction from the outer periphery of the small-diameter rollers 8 in the static position is minimized vertically upwardly and maximized vertically downwardly. In regard to the upper roller bearing 6, the small-diameter rollers 8 are put in rolling contact with the upper surface of the middle plate 5 and the large-diameter roller 9 is put in rolling contact with the lower surface of the upper plate 2. In regard to the lower roller bearing 7, the small-diameter rollers 8 are put in rolling contact with the upper surface of the lower plate 4 and the large-diameter roller 9 is put in rolling contact with the lower surface of the middle plate 5. The upper surface of the middle plate 5 and the upper surface of the lower plate 4 are provided with linear grooves 10 and 11, respectively, and the large-diameter rollers 9 of the upper roller bearing 6 and the lower roller bearing 7 are lodged in these linear grooves 10 and 11 in a non-contact manner.
When the upper and lower roller bearings 6 and 7 are in their static positions, the large-diameter roller 9 places its center of rotation P2 just below the center of rotation P1 of the small-diameter rollers 8 under a load from the superstructure 1 side, so that the roller 9 stably bears the above load in a state in which the quantity of protrusion thereof from the peripheral surface of the small-diameter rollers 8 is upwardly minimized and downwardly maximized.
When the substructure 3, and hence, the lower plate 4 fixed to this is shaken, for example, leftward in FIG. 12 by an earthquake, the middle plate 5 also moves leftward in the figure via the lower roller bearing. With this movement of the middle plate 5, the small-diameter rollers 8 of the upper roller bearing 6 put in contact with the upper surface of the middle plate 5 rotate in the clockwise direction around the center of rotation P1 by an angle corresponding to the quantity of movement of the middle plate 5. At the same time, the large-diameter roller 9 integral with the small-diameter rollers 8 also rotates in the clockwise direction. In this stage, the center of rotation P2 of the large-diameter roller 9 revolves around the center of rotation P1 of the small-diameter rollers 8. Therefore, it comes to be located diagonally below, just beside, or diagonally above the center of rotation P1 of the small-diameter rollers 8. Consequently, a turning moment produced by an imaginary arm having a length of a perpendicular extended from the center of rotation P2 to a vertical line passing through the center of rotation P1 acts on a point of contact of the large-diameter roller 9 with the upper plate 2. This turning moment acts as a restoring force, so that the upper roller bearing 6 is restored to its original static position. When the direction of shake of the substructure 3 is perpendicular to the sheet of the figure, the lower roller bearing 7 operates similarly to the aforementioned upper roller bearing 6. Thus, this base isolator is able to have an excellent restoring ability by the operations of the mutually eccentric rollers 8 and 9.
As a method for putting the base isolator as shown in FIG. 12 into practical use, a lower base frame, a middle base frame and an upper base frame provided with members which operate as the lower plate 4, the middle plate 5 and the upper plate 2, respectively, are formed in identical dimensions and stacked on one another. The aforementioned lower roller bearing and upper roller bearing are interposed between these members at the four corners, thereby forming four base isolation mechanisms. As a result, a base-isolated pedestal or plinth for a showcase is provided.
FIG. 13 is a plan view of the middle base frame. As clearly shown in this figure, between adjacent two upper roller bearings 6a is extending one continuous middle plate 5a in a direction in which these roller bearings rotate. Further, between adjacent two lower roller bearings (not shown) is extending one continuous middle plate 5b in a direction in which these roller bearings rotate. That is, the middle plates for two base isolation mechanisms M located on an imaginary straight line are connected with each other into a unitary plate. As is obvious, the middle plates of the base isolation mechanisms arranged in a plurality of places have been conventionally provided by a common long plate.
However, it has been discovered that the following problems occur when providing the middle plates of the plurality of base isolation mechanisms by a common plate, i.e., when their middle plates are connected with each other.
(1) Because the middle plates of the plurality of base isolation mechanisms are not independent of each other, the middle plate cannot freely move according to the rotation of the corresponding roller bearing in each base isolation mechanism. In other words, the middle plate has a low degree of freedom of movement.
(2) When the superstructure is deflected or bends with the load placed on it, the superstructure disadvantageously rests against the center portion of the middle plate in the lengthwise direction.
(3) The length of the middle plate depends on the planar size of a place to be equipped with the base isolator, or the size of the bottom surface of the superstructure. Therefore, the length of the middle plate is required to be changed according to the base isolator installation area. Therefore, the middle plate used in a certain installation place cannot be used in the other installation places. Thus, the apparatus lacks flexibility of use. Furthermore, when the installation place of the apparatus is broad, it is required to increase the dimensions of the middle plate. This requires material to be increased, causing a cost increase.